The present invention generally relates to a method and system for detecting motion, more particularly, to motion detectors capable of estimating the direction of moving objects.
Motion sensors for security systems are well known in the prior art. Generally, these systems automatically trigger an alarm when a motion sensing device detects a moving object within a specific area. The most common type of motion sensing devices are infrared detectors or video display apparatus.
When an infrared detector is used in an alarm system, the system is able to detect almost any movement in a specific area. Infrared detector technology is best for applications where an alarm system is to detect the presence of an object. However, infrared sensors have limited capabilities when an alarm system is used for functions outside of detecting the presence of an object. For example, if an infrared sensor were used to monitor an entrance of a building, the alarm would trigger every time a person passed through the sensing area. In this application, the alarm system would only be useful in monitoring the building entrance at night or at times of low traffic. In a situation where an alarm system operator is only concerned with persons entering a building, and not concerned with persons leaving the building, the use of an infrared detector is undesirable because it cannot distinguish the direction of objects moving in its field of view.
Other alarm system designs involve the use of a video camera and a video processing means. This design can be used in a number of different ways. In one design, the camera can be used in conjunction with a video monitor. For example, the camera can be directed to a passageway and the video image of that passageway is simply displayed on the video display. This design requires an operator to constantly watch the activity displayed on the monitor at all times. Thus, this arrangement is beneficial for situations where an operator desires to record the activity captured by the video camera or in situations where an operator only requires low level of security. However, this design does not work well in situations where an operator requires a high level of security. A security breach may occur if the operator does not constantly observe the video display. A design involving a video recorder has other disadvantages when applied to alarm monitoring systems. For example, even if the monitoring system records the activity viewed by the camera, a security breach can only be detected at a later time when the video tapes are reviewed under close inspection.
Other designs of alarm systems utilizing a video camera may also include a video processing unit. In this case, the video camera sends an analog video signal to a video processing unit that digitizes the images and stores them as a series of video image frames. The processing unit then compares each digitized video image frame and detects changes in the contrast or the image brightness between each successive frame. Much like the infrared detector, these type of alarm systems have the capability to detect general motion and have the same disadvantages when used to monitor high traffic areas.
Given these shortcomings, there is a need for a motion sensing system that can determine the direction of motion in a field of view and does not require continual monitoring by a human operator.
The present invention provides an alarm system to automatically monitor activity in a predetermined area. When the system detects movement in a predetermined area, an alarm is triggered which ultimately notifies a system operator or allows the system to initiate some other automated activity.
The alarm system detects the direction of a moving object by receiving a number of video image frames from a video camera that is positioned to view an area of interest. A processing unit then calculates a center of mass from data in each of the video image frames and then determines if the position of the center of the mass is changing. If the center of mass has changed in the video image frames, an alarm is generated to indicate a movement. Filters may be added to the system to reduce the number of false alarms. Specifically, the alarms may only be generated if the system detects movement in a particular direction by more than a predetermined amount or if the moving object detected by the video camera is of a particular size.
The system comprises a video camera, a video display, and a processing unit. The processing unit comprises a bus interface that connects a central processor (CPU), computer memory (RAM and ROM), a video processing unit, a video output unit, and an input interface unit. The processing unit receives video signals from a video camera and digitizes each of the video image frames. By the method of the present invention, the CPU detects the difference between subsequent video frames and calculates the center of mass of the differences. If the center of mass moves by more than a predetermined amount, an alarm is generated.